A so-called plane-mirror interferometer is described, for example, in U.S. Pat. No. 4,752,133. It includes a light source, a first beam splitter, and a detection unit. A beam of rays emitted by the light source is split via the first beam splitter into at least one measuring beam and at least one reference beam. The measuring beam and the reference beam then propagate in a measuring arm and in a reference arm, respectively, until they are recombined at the beam splitter. In the measuring arm, a measuring reflector in the form of a plane mirror is disposed on a movable object to be measured. The reference arm has at least one reference reflector. With the aid of the detection unit, at least one distance signal is ascertainable from the recombined interfering measuring and reference beams with regard to the position of the object to be measured.
A polarization beam splitter cube is provided as a beam splitter in U.S. Pat. No. 4,752,133. As a reference reflector, a plane mirror is used that is situated between the measuring beams. The combined triple prism of the reference arm and measuring arm is adhered onto or wrung to the beam-splitter cube. In addition, the triple prism may also be placed spatially separate from the beam-splitter cube.
It is considered to be disadvantageous with regard to such interferometers that both the polarization beam-splitter cube and the triple prism can only be produced with great expense. Thus, in the case of the beam-splitter cube, the various sides must be aligned highly exactly relative to each other, especially if one or more triple prisms and possibly further optical components are disposed directly thereon. In the case of the spatially separate placement of beam-splitter cube and triple prisms, a likewise very costly low-drift mounting of these components is necessary. In addition, if great tilt tolerances of the object to be measured are demanded, the triple prism must have a relatively large construction.